Photo acoustic spectroscopy (PAS) is a widely used method for trace gas detection. It is based on the photo acoustic (PA) effect, i.e. the conversion of light to sound in all materials (solids, liquids and gases). It applies to many chemical compounds, and, unlike conventional absorption spectroscopy, its sensitivity scales inversely with the volume of the sampling chamber.
To date, limited research has been done to demonstrate the feasibility of a miniaturized photo acoustic sensor. In order to realize the advantage of photo acoustic sensor miniaturization, light sources and detectors of comparable size are required. In order to achieve spectrometric performances, lasers are typically suggested as light sources. In recent years a large effort has been paid to develop semiconducting lasers (VCSEL, DFB, QCL) for spectral ranges of interest.
A “classical” spectrometric PAS has, beyond the laser, a chamber and a detector. The detector is usually in a form of a microphone. To increase the sensitivity of the system, a non-resonant chamber may be replaced with a resonant-one that is designed to have a resonant frequency equal to the modulation frequency of the laser. Thus, the amplitude of the pressure wave generated inside the chamber is amplified by the chamber that works at resonance.
A second approach to increase the sensitivity is to replace the microphone with an acoustic resonant detector. A solution developed by researchers from Rice University uses a quartz tuning fork (QTF) from an electronic watch (QEPAS—Quartz Enhanced Photo Acoustic Spectroscopy) as a resonant detector. In such a system the laser (chosen to fit an absorption line of the gas to be detected) is additionally modulated at a frequency related to the resonant frequency of the tuning fork. As the laser beam passes between and transverse to the two prongs of the fork, the laser light is absorbed by the target gas which heats up and creates a pressure wave. These pressure waves impinge on the prongs of the QTF which start to move in opposite directions. These deflections are finally converted by a piezoelectric effect to a current with an amplitude proportional to the concentration of the target gas.
The advantages of QEPAS systems are its small dimensions, high sensitivity and the elimination of the detection chamber replaced by the gap between the prongs of the QTF. While this concept has been proven in a laboratory setting, it seems difficult or impossible to adapt it to large scale manufacturing and to reap the benefits of further miniaturization.